Field of the Invention
The present invention relates generally to a system and method for dewatering and stabilization system, and more particularly, to a system and method for in situ dewatering and stabilizing saturated coal ash and other coal combustion residuals or similar waste material deposits previously created from sluicing into an ash basin or similar waste impoundment.
Description of the Prior Art
Impoundments have been utilized in the mining industry and power industry for disposal of waste products generated from the mining and burning of coal prior to the 1960's. Coal fired power plants have historically used basins or impoundments to contain and store waste products generated by the coal combustion process. Ash basins were designed to settle the coal ash and other coal combustion residual (CCR) solids and decant the water to adjoining surface water through a discharge structure. Deposited CCRs are typically saturated and layered, the accumulated layers described as pond ash deposit. The water level within the constructed embankments and the pond ash deposit is described as the phreatic surface, which may be perched or connected to the local groundwater level in surrounding soils and bedrock. Accordingly, the components of an ash basin typically include free water on top of the pond ash deposits and ponded around the outlet structure, leachate within the saturated pond ash deposit, and pond ash solids. The CCR materials accumulate in a pond ash deposit and remain in an undrained, saturated condition that is typically very loose or soft.
Historically, ash basins were constructed at a time when groundwater monitoring requirements and groundwater protections were not routinely required. Environmental impacts from existing ash basins have typically occurred in the foundation soils and rock materials adjoining the basins. The environmental risks posed by existing impoundments typically involve the migration of water with concentrated heavy metals, sulfates, chlorides, total dissolved solids, or total suspended solids to adjoining surface water or groundwater.
During most of their useful life, ash basins remain passive facilities and only receive the work necessary to maintain the basin embankments and the flow path to the outlet structure. As the basin approaches its storage capacity, shallow dewatering, excavating and/or dredging activities have been used to relocate or stack pond ash to improve settlement and storage conditions within the basin. After excavation, the leachate drains from stacked pond ash leaving drier, unsaturated ash materials. The stacked ash is more dense and stable than the sluiced ash and supports heavy equipment loads. Conversely, the adjoining unimproved pond ash deposit is subject to failure under excessive static load conditions or could liquefy or spread laterally from vibration or earthquake conditions. Based on these observations and on laboratory testing of drained and undrained ash samples, consolidation and an increase in shear strength are the structural improvements needed for pond ash deposit stability.
The process treatment of the wastewater collected from an ash basin provides risk reduction for groundwater and surface water quality. Both passive and active methods for wastewater treatment to remove metals, total suspended solids and total dissolved solids have been successfully implemented in the mining and power industry. The water treatment methods to remove heavy metals from groundwater and coal wastewater have been successfully implemented for similar applications and new technologies are under development.
Existing CCR landfill overfills have been developed over historic pond ash deposits and provide added capacity for CCR disposal. For existing and new overfills, dewatered pond ash deposits improve the foundation conditions by consolidating the old pond ash deposits, which will increase the factor of safety for stability.
The current state of the art in dewatering ash basins includes excavating or dredging the pond ash, draining the leachate back to the pond, and moving the ash to a storage location. The pond ash is typically saturated when excavated and must be temporarily stacked to drain prior to loading the pond ash on trucks for hauling. Existing methods remove the leachate and ash together from below the phreatic surface to an elevated position where the leachate gravity drains from the pile, typically back into the basin. Dredging and pumping the ash-leachate slurry to process equipment for solid-liquid separation has been used to remove the pond ash deposit. With excavation or dredging, the existing methods disturb the pond ash deposit and remove the ash and leachate together. Unless the leachate accumulating in the pond during excavation is properly treated, environmental risks to surface water quality are increased during the excavation and drainage process.
The costs and emissions from an excavation or dredging operation to remove ash from an ash basin are significant. The weight of heavy construction equipment over saturated weak material and unstable conditions in the pond ash deposit is a general safety risk for conventional excavation on a pond ash deposit. In the mining industry there is a record of loss of life and equipment from these excavation activities. The safety risks are greater with increasing excavation depth.
The in situ treatment system for dewatering and stabilization of the present invention comprises a method to lower the phreatic surface with limited disturbance to the pond ash deposit for environmental risk reduction, remediation, and closure of the ash basin. The phreatic surface can be lowered and risks for contaminant migration are lowered when the leachate is collected and removed from the basin. The remaining wastes may avoid removal and transportation to an alternate site for disposal if the risks are reduced to an acceptable level.
The in situ treatment system also includes specialized wells to retain and filter solids from the recovered leachate. Controlling suspended solids from clogging the collector and damaging pump equipment is a key component of the in situ treatment system. The operation of the collectors may be concurrent with existing practices to manage leachate removal from the pond ash deposit and may be supplemented by adjoining groundwater recovery. The dewatered CCR solids will consolidate under their self-weight and will have improved strength and structural properties to support overfill of additional wastes or in situ closure. If necessary, dewatered CCR solids may be removed for secure disposal.
U.S. Pat. No. 6,351,900 discloses a shaft driven trencher that includes a mobile powered base, a boom assembly, a chain digging assembly, a drive assembly and a linkage. The boom assembly includes a first boom member pivotally mounted to the base at a first pivot and a second boom member pivotally mounted to the first boom member. The chain digging assembly has a plurality of buckets. The drive assembly operably interconnects the mobile base and the chain digging assembly, with the drive assembly including a drive shaft connected to the mobile base for powered rotation of the drive shaft. The drive assembly further includes a right angle gear unit with input and output shafts, with the right angle gear unit being pivotally mounted to the boom assembly for rotation about the output shaft. The linkage interconnects the right angle gear unit with the base and pivots the right angle gear unit during rotation of the first boom member. The linkage also maintains the input shaft of the right angle gear unit facing the first pivot.
U.S. Pat. No. 6,390,192 discloses an integral well filter and screen and method for making and using same. A first embodiment includes a perforated cylindrical well screen with a filter assembly in its interior. The filter assembly includes a pliable filter material bonded to a filter support. The filter support is a plastic lattice rigid enough to hold the shape of the filter assembly and keep the filter material in contact with the inner surface of the well screen, yet deformable enough that the filter assembly can be formed into a shape consistent with the interior of the well screen. A second embodiment includes a well screen with a filter made of a rigid, porous material placed inside the well screen. Because the filter is rigid, it needs no filter support. The method for the first embodiment includes bonding the filter support to the filter material, forming the filter assembly into a cylindrical shape complementary to the inner surface of the well screen, securing abutting edges of the filter assembly to maintain its shape and rigidity, and ensure complete filtration, and inserting the filter assembly into the well screen. The manufacture of the filter in the second embodiment is similar, but does not include the bonding of a filter support to the rigid porous material.
U.S. Patent Publication 2014/0255100 discloses a geosynthetic composite for filtration and drainage of fine-grained geomaterials that includes a geonet and a geotextile. The geotextile has a nonwoven fabric layer and a woven fabric layer, with the nonwoven fabric connected by needle-punching to the woven fabric whereby fibers of the nonwoven fabric extend through and beyond the woven fabric, and the woven fabric and fibers of the nonwoven fabric extending through the woven fabric are bonded to one side of the geonet.
The prior art to date does not disclose a system and method for dewatering and stabilizing coal combustion residuals or similar waste material deposits previously created from sluicing into an ash basin or coal combustion residual impoundment. None of the prior art can be combined in a way to suggest these necessary modifications. There is no teaching, suggestion, or motivation that would have enabled a person of ordinary skill in the art to modify any prior art to arrive at the present invention.
It is a primary object of the present invention to provide an in situ treatment system and method for dewatering and stabilization that removes leachate from the pond ash deposit and pond in situ.
Another object of the present invention is to provide an in situ treatment and method that reduces environmental risks associated with pond leachate.
Still another object of the present invention is to provide an in situ treatment system and method that consolidates and stabilizes pond ash solids.
Still another object of the present invention is to provide an in situ treatment system and method that allows the closed pond to be developed as an overfill landfill for added waste disposal.